1. Field of the Invention
The present invention relates to a coherent dual-channel QPSK (quadrature phase shift keying) modulator/demodulator for a CDMA (code division multiple access) system, and modulating/demodulating methods therefor, in which, the crosstalk interference between an I-channel and a Q-channel are eliminated by an orthogonal quadrature spreading method, and the channel parameter estimation errors are minimized, thereby improving receiving performance.
2. Description of the Prior Art
In a DSSS (direct sequence spread spectrum) communication system, the QPSK modulating and demodulating methods can be classified into a balanced QPSK and a dual-channel QPSK.
The IS-95 CDMA system proposed by Qualcom employs the balanced QPSK and the wideband CDMA system proposed by OKI employs a variety of the dual-channel QPSK.
In the wideband CDMA system, the same Walsh codes are used both on the I-channel and on the Q-channel of the modulator and the demodulator W.sup.I (n)=.sup.Q W(n)!, and therefore, the following crosstalk interferences occur in the demodulator output: ##EQU1##
Where N is spreading factor, .alpha..sub.j,k (n) is the amplitude of n-th path corresponding to j-th user, .alpha..sub.j,k (n) is the estimated value of .alpha..sub.j,k (n), .sqroot.(E.sub.c,j) is the chip energy of j-th user, X.sup.I.sub.j (n) is the input data of I-channel corresponding to j-th user, .phi..sub.j,k (n) is the phase of n-th path corresponding to j-th user, and .phi..sub.j,k (n) is the estimated value of .phi..sub.j,k (n), V.sup.I.sub.N (n) is the psudonoise which appeares in I-channel, while V.sup.Q.sub.N (n) is the psudonoise which appeares in Q-channel. And, the term X.sup.I.sub.j (n)sin(.phi..sub.j,k (n)-.phi..sub.j,k (n)) indicates the crosstalk interference which appears in the I-channel, while the term X.sup.Q.sub.j (n)sin(.phi..sub.j,k (n)-.phi..sub.j,k (n)) indicates the crosstalk interference which appears in the Q-channel. In the case of imperfect estimation, the interference increases, thereby deteriorating the performance.